ARM: dts: stm32: support child mfd cells for the stm32mp1 TAMP syscon

The stm32mp1 TAMP peripheral has 32 backup registers that survive
a warm reset. This makes them suitable for storing a reboot
mode, which the vendor's kernel tree is already doing[0].

The actual syscon-reboot-mode child node can be added by a board.dts or
fixed up by the bootloader. For the child node to be probed, the
compatible needs to include simple-mfd. The binding now specifies this,
so have the SoC dtsi adhere to it.

[0]: https://github.com/STMicroelectronics/linux/commit/2e9bfc29dd



Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>